Maui has a wide array of water activities and beautiful beaches. Pack several swim suits, your sunglasses and a beach towel or two.
Sunscreen is an absolute necessity; you are much closer to the equator here and will burn very quickly, especially in the water. Look for ecologically safe sunscreen that won't harm the fish and plants in the ocean. It might be labeled as dolphin-friendly.
If you're going to be at the beach for more than an hour, you will want an umbrella. This allows you to pop back under the shaded area when you get too hot. A few sand chairs are a good idea, as well.
Snorkel gear can be rented or purchased on the island, but you might find it cheaper in your hometown. If you have your own wet suit, you might want to bring it along as well.
Don't forget to bring a good book or two. You will endure a flight of five hours or more to get to the island from the mainland, and there will be plenty of time to lounge about and read while you are on the beach.
You might not think of Maui as being a destination for hikers, but you will find some of the most beautiful places on the island by taking short (or long) walks. The road to Hana has trails along the entire length where you can hike in and find stunning waterfalls and crystal-clear pools, and if you hike in far enough you might find yourself alone. Hiking sandals are a good option so that you don't have to remove your socks once you get there, and you can splash along in the river bed without getting too squishy.
You will also find a great hike on Haleakala, a popular destination for hikers who want to see an amazing sunrise. Here, a standard pair of hiking boots is nice to have to avoid cutting your feet on the volcanic rock, but the same hiking sandals will work as well.
Bring along a hat and a canteen for hiking.
There is plenty of shopping to be found on Maui and lots of restaurants and entertainment.
For shopping, make sure you have a comfortable pair of shoes or sandals. A large tote bag is nice so that you do not have to juggle several packages.
Typical dress is very casual on the island, but you might want to dress up a bit to go out at night. A nice sundress for women and khaki pants and buttoned shirt for men are good attire for evening shows, dancing and luaus. Bring a lightweight sweater or jacket--evenings can get a bit cool.
